1. Field of the Invention
The present invention relates to a molding die, and more specifically to a glass molding die and renewing method thereof.
2. Description of the Related Art
Molding dies disclosed by Matsushita Electric Industrial Co., Ltd. show outstanding lifetime performance at high working temperatures resulting from addition of a third element in a noble metallic alloy to prevent grain growth. For example, a lifetime of the molding die disclosed in JP63103836 by Aoki et al. can be 2,000 cycles or more at working temperature of approximately 800° C. resulting from use of an Ir—Re—C alloy as a passivation film directly on a tungsten carbide substrate with no intermediate layer therebetween. However, noble metal layers such as Ir—Re based alloys can be removed by only one method, grinding, resulting in long cycle time when renewing the molding die.
A diamond-like carbon (DLC) layer on a holding die can be completely removed by oxygen plasma, followed by grinding of the molding die to renew, such that renewing thereof is simple. However, when the DLC layer is thicker, it may peel by internal stress thereon. Further, when the DLC is thinner, elements in the substrate may diffuse to the surface of the DLC layer, resulting in discoloration and reaction with glass, thereby deteriorating the stripping properties thereof. Hirota et al. disclose a molding die having a DLC layer acting as a passivation film of a substrate and an intermediate layer of silicon carbide or silicon nitride therebetween in JP1320233. Uno et al. disclose a molding die in JP6191864, having a DLC layer, formed by sputtering, acting as a passivation film of a substrate, and two intermediate layers, silicon carbide and ion-implanted DLC, therebetween.
Hirota discloses a molding die in JP 11079760, having a DLC layer acting as a passivation film of a substrate and a consumable β-silicon carbide acting as an intermediate layer about 3 to 50 mm thick therebetween, which can be renewed by completely removing the DLC layer using oxygen plasma and grinding the β-silicon carbide layer to gain a predetermined shape. The molding die provides good renewability, but the β-silicon carbide layer thereon is thick enough that formation thereof requires hundreds of hours, a time-consuming and costly process.